Low-pressure arc discharge lamps and, in particular, fluorescent lamps are well known; see IES Lighting Handbook, 1981 Reference Volume, Section 8.
Herein, the terms "end-of-life" and "end of the useful life" of a low-pressure arc discharge lamp are defined as that time when the electron-emissive material on one electrode filament has been depleted so that the arc discharge destroys the filament and strikes other parts of the electrode structure.
Low-pressure arc discharge lamps, especially those designed for operation at high current loading, such as very high output (VHO) lamps, sometimes fail by causing the fracture of the glass envelope. It is believed the sequence of events leading to such failures is as follows. At the end of the useful life of the lamp, the electron-emissive material on one of the electrode filaments becomes depleted. When such depletion occurs, the arc discharge strikes other components of the electrode structure and, in particular, the arc strikes the lead-in wires supporting the electrode structure. The lead-in wires are heated by the arc to the point where the wires soften and bend. Subsequently, the lead-in wires and the electrode structure sags and comes in contact with the glass envelope. The severe heat generated by the arc and the heated electrode structure cause the glass envelope to fracture.
Various internal structures for low-pressure arc discharge lamps have been proposed which cause the lamp to fail without fracture of the glass envelope. Such structures are suggested in the following references.
U.S. Pat. No. 3,265,917, issued Aug. 9, 1966, to Ray, discloses a structure comprising a wire or conductive coating electrically connected to the inside portion of the electrode structure and extending to a thin-walled portion of the stem. Upon depletion of the electron-emissive material on the electrode filament, the arc strikes and follows the conductive path reaching the thin-walled portion of the stem. The heat generated by the arc and the heated conductor softens and melts the thin wall of the stem to the point where the hermetic seal is lost. The introduction of the external atmosphere into the lamp extinquishes the arc discharge and renders the lamp inoperable.
Japanese patent publication No. 44-15840, dated July 14, 1969, by Sometani et al., discloses a structure comprising a strip of aluminum powder coating over a portion of the stem and in electrical contact with one of the inside lead-in wires.
U.S. Pat. No. 3,562,571, issued Feb. 9, 1971, to Evans and Morehead, discloses a structure for an amalgam-regulated low-pressure arc discharge lamp which serves the dual function of providing an auxiliary. source of amalgam for faster warm-up and providing for extinguishment of the arc discharge at the end of the useful life of the lamp. The end-of-life structure comprises a notched yoke of wire mesh or sheet metal electrically connected to an inside lead-in wire and clipped onto the stem press near an exterior portion of the lead-in wire.
U.S. Pat. No. 4,105,910, issued Aug. 8, 1978, to Evans discloses a structure providing for an auxiliary source of amalgam and for end-of-life extinguishment of the arc. This structure comprises a coating of a suitable amalgamative metal on portions of the stem press and the inside lead-in wire about the point where the lead-in wire emerges from the stem press.
A copending U.S. patent application, Ser. No. 349,722, filed Feb. 18, 1982, by Young and Sadoski, assigned to GTE Products Corporation, discloses a combination getter and end-of-life structure comprising a coating of aluminum and zirconium; such coating being applied to selected areas of the stem, having electrical contact with at least one inside lead-in wire, and extending to a thin-walled portion of the stem's body.
In the aforementioned references, the end-of-life extinguishment of the arc discharge is based on the principle of breaking the lamp's hermetic seal through a thin-walled portion of the stem. An internal structure or device having the ability to extinguish the arc discharge at the end of the useful life of a low-pressure arc discharge lamp without causing the loss of the lamp's hermetic seal is desirable and would be an advancement of the art.